This invention relates to liquid crystals and more particularly to a method for aligning a polymeric film with polarized light without the need to mechanically rub the substrate surface, and to a method for obliquely aligning liquid crystals therewith. This invention also relates to a liquid crystal cell having an optically active compound-containing polymeric film obliquely aligned by polarized light as an aligning layer and a method of manufacturing the same.
Among flat panel display devices, a liquid crystal(LC) display device is a display device using an optical rotation property according to the orientation of liquid crystal molecules. It has a structure in which upper and lower substrates are spaced from each other by a spacer and the space between the substrates is filled with liquid crystal. Here, each of the upper and lower substrates has an aligning layer formed therein.
So as to make a liquid crystal device with uniform brightness and high contrast ratio, there is the need for aligning liquid crystal molecules in a same direction. This is because physical constants of liquid crystal varies with the orientation of liquid crystal molecules and thus a difference in the response to external force such as electric field etc., occurs. Therefore, the study on a method of controlling the alignment of liquid crystal molecules in macroscopic scales device has been widespreadly performed.
There are a homogeneous alignment, an oblique alignment, and an homeotropic alignment in basic alignment types.
Here, in the homogeneous alignment, the long axis of the liquid crystal molecule is parallel with the substrate surface and a pretilt angle .theta. is thus 0.degree.. In the oblique alignment, the pretilt angle .theta. is 0.degree.&lt;.theta.&lt;90.degree.. In the homeotropic alignment, the pretilt angle .theta. is 90.degree.. The pretilt angle 0 is measured as the angle that the long axis of the liquid crystal molecule makes with the surface of the substrate. In this regard, the size of the pretilt angle .theta. is an important variable which has an effect on a response time of the liquid crystal to external force. In the case of a twisted nematic liquid crystal cell, when the pretilt angle is 0.degree., two domains, namely, a rightward twisting domain and a leftward twisting domain in the liquid crystal, occur. Accordingly a disclination occurs and contrast ratio is therefore attenuated.
In general, so as to align the liquid crystal poured between upper and lower substrates in a same direction, the alignment layers are formed on the substrates, respectively. Examples, the conventional method of forming the alignment layer for liquid crystal includes a SiO2 depositing method, a coating method, and a rubbing technique using an organic polymer, etc.. The rubbing technique using a polyimide polymer film as an aligning layer has been most widely applied.
The method of aligning the liquid crystal in accordance with the rubbing technique comprises coating polyimide films as aligning layers on the surfaces of upper and lower substrates, respectively which TFT arrays and color filters, respectively are formed; curing the coated polyimide films; rubbing the polyimide films with rubbing cloth, rabbit fur, etc., to form grooves in a same direction on the films; and aligning liquid crystalline molecules in a same direction along the grooves. Here, the alignment of a liquid crystal is controlled according to the application of LCD devices or the direction of main viewing angle.
This technique has inherent problems, however, in that rubbing creates both electrical damage and mechanical damage in TFT arrays placed below an aligning layer in a liquid crystal cell. In addition, owing to static charges and dust particles, the lifetime of LCD devices is shortened.